1. Field of the Invention
The present invention relates to a method, system and gateway device for enabling interworking between an Internet Protocol (IP) network, such as an IP Multimedia Subsystem (IMS) network, and a circuit-switched (CS) network, such as a Global System for Mobile Communication (GSM) network.
2. Description of the Related Art
In order to achieve access independence and to maintain a smooth interoperation with wired terminals across the Internet, an IP multimedia subsystem (IMS) core network as specified e.g. in the 3GPP (Third Generation Partnership Project) specification TS 23.228 has been developed to be conformant to IETF (Internet Engineering Task Force) “Internet Standards”. The IMS enables network operators of mobile or cellular networks to offer their subscribers multimedia services based on and built upon Internet applications, services and protocols. The intention is to develop such services by mobile network operators and other third party suppliers including those in the Internet space using the mechanisms provided by the Internet and the IMS. The IMS thus enables conversion of, and access to, voice, video, messaging, data and web-based technologies for wireless users, and combines the growth of the Internet with the growth in mobile communications.
IETF and 3GPP are working on a Session Initiation Protocol (SIP) conferencing service. The goal is to define how conferencing type of services can be established between 3GPP compliant SIP terminals. Simultaneously with this work, another study is underway as to how the interworking between 3GPP IMS and legacy circuit-switched (CS) core network domains can be achieved. A cellular network, i.e. a Public Land Mobile Network (PLMN) can be regarded as an extension of networks with CS domains and packet switched (PS) domains within a common numbering plan and a common routing plan. The PLMN infrastructure is logically divided into a core network (CN) and an access network (AN) infrastructure, while the CN infrastructure is logically divided into a CS domain, a PS domain and an IMS. The CS and PS domains differ by the way they support user traffic. These two domains are overlapping, i.e. they contain some common entities. A PLMN can implement only one domain or both domains. In particular, the CS domain refers to the set of all CN entities offering CS type of connections for user traffic as well as all the entities supporting the related signaling. A CS type of connection is a connection for which dedicated network resources are allocated at the connection establishment and released at the connection release. The PS domain refers to the set of all CN entities offering PS type of connections for user traffic as well as all the entities supporting the related signaling. A PS type of connection transports the user information using autonomous concatenation of bits called packets, wherein each packet can be routed independently from the previous one. The IMS domain includes all CN elements for provision of IP multimedia services including audio, video, text, chat, etc. and a combination of them delivered over the PS domain.
So far, conferencing has been considered from IMS point of view where simultaneously communicating parties are IMS subscribers with IMS subscription. This scope enables the full end to end use of SIP between the participants. SIP is an application-layer control protocol for creating, modifying and terminating sessions with one or more participants. These sessions include Internet multimedia conferences, Internet telephone calls and multimedia distribution. Members in a SIP session can communicate via multicast or via a mesh of unicast relations, or a combination of these. In the full end to end use of SIP no specific requirements for interworking between non-SIP users, not having a SIP capable terminal equipment, have been considered.
However, interworking between IMS and CS domains and between SIP-based conferencing and CS domains has not been considered yet. Basically this would allow conferencing participants to be selected also from a CS domain, i.e. regular telephone users such as users of second or third generation mobile networks, fixed public-switched telephone networks (PSTN) or fixed integrated services digital networks (ISDN).